1. Field of the Invention
This invention relates to a novel liquid crystal compound useful for display elements and a chiral smectic liquid crystal composition comprising the above liquid crystal compound and an optically active compound.
2. Description of the Prior Art
At present, as to liquid crystal display elements, TN (Twisted Nematic) type display mode has been most broadly employed, but as far as the response speed is concerned, such TN type display elements are inferior to emissive type display elements (such as those of electroluminescence, plasma display, etc.). Although various improvements in this respect have been attempted, it appears that possibility of the improvement to a large extent has not been left behind so much. Thus, various liquid crystal display devices based on a different principle from that of TN type display elements have been attempted. As one of such devices, there is a display mode utilizing a ferroelectric liquid crystal (N.A. Clark et al: Applied Phys. lett., 36, 899 (1980)). This mode utilizes the chiral smectic C phase (SC* phase), chiral smectic F phase (SF* phase), chiral smectic I phase (SI* phase), chiral smectic G phase (SG* phase) or chiral smectic H phase (SH* phase) of the ferroelectric liquid crystal, and those having these phases in the vicinity of room temperature are preferred.
These chiral smectic liquid crystal materials may be obtained by blending a plurality of single substances exhibiting chiral smectic phases by themselves, but it has been known that the above liquid crystal materials may also be constituted by adding an optically active compound, preferably an optically active liquid crystal compound, more preferably a chiral smectic liquid crystal compound to an achiral smectic liquid crystal (such as those having SC phase, SF phase, SI phase, SG phase, SH phase or the like).
As to liquid crystal compounds exhibiting smectic phases, various kinds thereof have already been known, but as to whether or not the chiral smectic liquid crystal materials obtained by adding thereto an optically active compound exhibit fully superior performances in ferroelectric liquid crystal displays, no ultimate evaluation has yet been obtained. This is due to the fact that liquid crystal displays utilizing ferroelectric properties have not yet been technically completed. Thus, in the present status, it is necessary to examine various kinds of new smectic liquid crystal compounds.